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Those Blasted Plastids.......By Jim Hawes, Oakland Maryland For the last year or so I have been studying the subject of chlorophyll quite intensively. This document represents a summary ofwhat I have learned. I have found it an intriguing subject but not very well understood by many people. It is important to understand chlorophyll because if we understand the basis for color in hostas, it helps us become better gardeners. I will try to keep my summary as simple as possible. This is not about scientific research, that I am writing. It is about generally accepted scientific knowledge written in any good textbook on plant physiology and available to be read and interpreted by anyone willing to look it up in a library. Chlorophyll is a generic name for green pigments in plant cells....a substance that absorbs visible light primarily in the red, violet and blue regions of the light spectrum. There are several kinds of chlorophyll with chlorophyll a being the most important for light dependent reactions in the complex photosynthesis processes. Chlorophyll a and b exist in plastids in cells of higher plants while chlorophyll c,d and e are present only in algae. Photosynthesis is the process of converting light energy into chemical energy which can only be performed by plants. All life on earth depends upon the ability of plants to photosynthesize simple sugars which are the basic source of food from which all other forms of food originate. A chlorophyll molecule is made up of carbon and nitrogen atoms joined in a complex ring with an atom of magnesium located in the center of the ring. The molecule has a long chain of 20 carbon atoms making up an alcohol "tail" attached to the ring. Each kind of chlorophyll may vary somewhat in its molecular structure giving it slightly different chemical and physical properties. To further understand the structure and function of chlorophyll, imagine these molecules being stacked on top of each other like saucers with the tail of each molecule extending into two membranes that cover the stack of saucers. The stack of saucers is called a thylakoid and functions somewhat like a battery creating electrical current during photosynthesis. Several stacks of thylakoids exist inside a membrane covered structure called a grana which, along with several other grana (perhaps as many as 50), are located inside a typical chloroplast. The thylakoid space represents the region inside the grana where complex chemical/physical reactions associated with photosynthesis occur. The colors of hosta leaves as percieved by the human eye, are determined by whether the various pigments contained in cells reflect or absorb the various wave lengths of light in the visible spectrum. Green leaves appear green because green light is reflected from hosta leaves, not absorbed. The peak of absorption by chlorophyll a is at 700 nanometers employing a spectrophotometer to make such measurements. I am familiar with some of the Beckman Spectrophotometer Models having worked in an analytic laboratory as a Graduate student at the University of Maryland in a research project measuring chlorophyll, carotinoids and other pigments in plant tissues. Who would have dreamed some fifty years ago that such research work would have a direct application in understanding color pigments in hosta plants in 1997. Chlorophyll b has a slightly different molecular structure from that of chlorophyll a which gives it a capability to absorb light of different wave lengths from that of chlorophyll a. Chlorophyll b is called an accessory pigment, being yellow- green in color while chlorophyll a is bright green as perceived by our eyes. The more chlorophyll a there is in a cell, the brighter green the color of the cell and its constituent tissue. There usually is a ratio of 3 to 1 of chlorophyll a and chlorophyll b in most photosynthesizing cells. Chlorophyll appears to have three functions:
P700 is a type of chlorophyll a molecule which has a specific function in the complex Photosystem I Stage (light dependent stage) of photosynthesis. This type of chlorophyll a not only absorbs light itself, but also accepts "protons" (which are H+ electrons transferred from many types of "excited" chlorophyll and accessory pigments) and continues transferring them across hydrogen ion gradients which exist on the thylakoid membranes to primary acceptors. P700 is the only type of chlorophyll a molecules which can perform this transfer functions in Photosystem I Stage . P680 is another distinct chlorophyll a molecule which is indispensible in the light independent stage of Photosystem II Stage. It is called P680 because its peak of absorption is at 680 nanometers as measured by a spectrophotometer. Each photosystem contain 200 to 300 pigment molecules which serve as antennae to capture solar energy. After this light energy is absorbed, it is passed from one molecule to another until it arrives at the P700 and P680 molecules located in "primary reaction centers" where it gives up these excited electrons to electron acceptors. This electrical energy is the driving source of energy for the overall complex photosynthesis systems. Only P700 and P680 have the capacities to receive and transfer electrons in these photosystems. The rate of photosynthesis depends to a certain level on light intensity. However, because only a small portion of chlorophyll molecules in a thylakoid are excited at any one time, light is not necessarily the limiting factor for increasing the rate of photosynthesis. Under conditions of high light, other factors ( such as the amount of carbon dioxide available) may become more important than light intensity in increasing the photosynthetic rate. This principle is well known by commercial flower growers who often intentionally produce additional carbon dioxide gas in greenhouses to increase growth rates via increased photosynthesis. The overall system of photosynthsis is complex and diverse. I will not attempt herein to "explain" it to readers. Generally accepted models of all phases of the complex , inter-related systems have been described in detail. For those readers further interested, I refer you to information sources given later.* Our discussion now centers on plastids and pigments within the plastids. Plastids are small structures known as organelles which exist inside the cytoplasm of many cells of a typical leaf. There may be as many as 100 plastids inside some cells. They may exist in several forms. Plastids may possess other accessory photosynthesis pigments such as the carotinoids. Carotinoids are any class of more than 300 known fat soluable yellow, orange, red or purple pigments in the form of carotenoprotein complexes located in grana of plastids and usually as four molecules of "excited"peridinin around each chlorophyll molecule. Because of their molecular structure, carotinoid pigments are able to absorb wave lengths of light that are different from those absorbed by chlorophyll pigments. It is for this reason that they are known as "antenna or accessory photosynthetic pigments". Typical carotinoid pigments are lycopene, alpha carotene,beta carotene, astaxanthin and fucoanthin. These are usually grouped as xanthophylls and carotins. Forms of plastids which exist in plant cells include protoplasts, chloroplasts, leucoplasts, amyloplasts, proteoplasts, chromoplasts and etioplasts. Each are briefly discussed. Protoplasts are small, undifferentiated, immature plastids in meristematic cells. They are capable of rapid division and can increase to significant numbers. Upon growth and development, they may differentiate into other specific plastids with specific functions. Chloroplasts have large numbers of thylakoids, containing a myriad of precisely oriented chlorophyll molecules, permitting the generation of minute electrical currents which power the photosynthesis processes. Leucoplasts are chloroplasts which possess only limited numbers of grana comprised of stacks of thylakoids with chlorophyll molecules, thus, cells and tissues in which they exist often appear white or colorless. Amyloplasts are colorless also and usually occur in storage tissues. They contain starch grains composed largely of amylose type starch. Much of the synthesis of proteins which occurs in cells of leaf tissue occurs in proteoplasts. These plastids have large masses of protein comprised in complexes with starch. Chromoplasts contain green pigments but are often masked or replaced by carotenoid pigments of various kinds. Etioplasts, which occur under conditions of darkness contain large surfaces of colorless membranes and grana. When exposed to light, they usually develop into other types of plastids. It appears that plastids in their various forms can change morphologically when conditions within and outside the cells change. Amyloplasts can change into chloroplastrs with increased light. Chloroplasts can be modified into chromoplasts and chromoplasts can "re-green" into chloroplasts. Kevin Vaughn has reported that chloroplasts can become "dilated" and thus change form, color and function. Some investigators have suggested that such modifications in the structure in the plastids, affecting grana and thylakoid populations, are a type of self-destruction of the plastids to allow the plants to adapt to changing environmental circumstances. Such changes in structure and function of plastids are most likely controlled by both genetic and environmental influences in an interactive manner. I have proposed elsewhere* * that color in hosta leaves is largely an expression of plastid populations, types and distribution patterns in leaves. The color changes that occur in hosta leaves is probably due to the ever changing forms and numbers of plastids within critical cells of leaves. These color changes are highly influenced by changing environmental conditions, operating , of course, within the genetically programmed potential for such changes to occur. A discussion of several examples of such color changes follow. There are several predictable seasonal color changes which hosta gardeners are familiar with. These have been identified as being genetically influenced but only seasonal in duration. I refer to the changes known as albescence (changing to white), leutescence (changing to cream or yellow) and viridescence (changing to green). These changes are not to be confused with permanent changes in tissue color which are commonly known as "sports". However, even within sports,predictable color changes occur, thus, there may be some genetic-based permanent characteristics repeated year after year that represent examples of seasonal color changes involving the albescence, leutescence and viridescence phenomena. In discussing plastids and pigments, the term carotinoid plastids has been used by some authors. In this discussion the more general terms chloroplasts or plastids will be used to describe plastid forms which have associated accessory carotinoid pigments attached to chlorophyll molecules. If the population of chlorophyll molecules decreases significantly ( such as when chloroplasts become dilated, change form, self destruct and/ or become metabolized within the plant tissues), the concentration of carotinoid pigments then appear to predominate and become much more noticable. We call this phenomenon leutescence. When concentration of carotinoid pigments are low, the same phenomenon might be called albescense. When the reverse phenomenon occurs and chloroplast populations increase to levels sufficient to mask carotinoids or non pigmented tissues, we call this phenomenon viridescence. A given tissue within a hosta leaf might change during the season to all three types of seasonal color variability depending upon genetic propensities and environmental conditions.H. 'Whirlwind ' is such an example, being white centered, then changing to leutescent and viridescent later in the season. Aside from predictable seasonal color changes that may occur, there are many other temporary color changes noticed by all gardeners. The most common change is that related to nitrogen availability and its influence on hosta color. Since each molecule of chlorophyll contains four atoms of nitrogen in its principle ring-like structure, any deficieny of nitrogen is translated into a deficiency of chlorophyll pigments of many types in affected cells. There are mechanisms within the plants to compensate for short term deficiencies....cells may borrows nitrogen from other cells nearby as chlorophyll in these cells becomes self-destructed. The chlorophyll is "metabolized" and its constituent atoms are moved elsewhere within the plant...usually to the most rapidly growing parts of the plant,the meristems. Since magnesium is an integral, indispensible atom of the chlorophyll molecule, chlorophyll may become deficient if magnesium becomes deficient. Application of additional magnesium sulfate to hosta garden soils is now a rather common practice to insure that adequate magnesium is present to allow synthesis of high populations of chlorophyll-rich plastids in cells of hosta leaves. Environmental conditions which hinder the maximum uptake of the constituent nutrients nitrogen and magnesium include low levels of these nutrients in the soil, low moisture content in soils, high light intensities and high evapotranspiration rates. Any of these inter-related factors, if detrimental, will affect other factors and influence uptake of nitrogen and magnesium, thus affecting chlorophyll and plastid populations within plants. Resulting color changes should be considered temporary and even reversible when changes in the availability of nitrogen and magnesium occur. This principle applies in some degree to other macro and micro nutrients as well, but not to the extent as those nutrients which are constituents of chlorophyll. gardeners have learned several management practices to maximize color intensities, to encourage color contrast between tissues of different colors and to optimize other color features in hostas. We put our green hostas in shade, we position our yellows in more sunny locations, we place our whites often in full sunlight, always looking for the perfect growing environment to maximize beauty expressed through color. We use more art than science in making these garden decisions. Perhaps now we know a little more about the science of placing plants in their best environment. I have only touched lightly on a very complex subject.....coloration in hosta foliage. Now I and must end this discussion somewhere. Here is a good place. In summary, the relative population of green chloroplasts with differing chlorophyll pigments, the amount and kinds of associated yellow or orange carotinoid pigments and the population of colorless leucoplasts in cells give the various tissues and organs within hosta plants their characteristic colors of green, yellow and white in various distribution patterns. These patterns are highly variable in hostas as we see in the hundreds of types of variegation in hosta leaves. Hostas are unique in their color characteristics. I suppose that's why we grow them and love them. ------------------ * Photosynthesis: Capturing Energy, Chapter 8, Biology, Second Edition, Saunders College Publishing, Philadelphia, 1989 **Hawes, J., Using an Artist's Palette to Classify Hosta Sports, The Hosta Journal, Vol.27. No.1, 1996 |
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